Sputtering, a physical-vapor-deposition technique, is utilized in many industries to deposit thin films of various materials with highly controllable composition and uniformity on any of a variety of substrates. In a sputtering process, a sputtering target of the material to be deposited (or a component thereof) is subjected to bombardment by energetic particles (e.g., plasma particles), which thus eject atoms of the target material toward the substrate, on which they are deposited. Conventional new (i.e., unused) planar sputtering targets have flat round or flat quasi-rectangular shapes. For example, FIG. 1 depicts a new sputtering target 100 idealized as a rectangular prism. (In reality, planar sputtering targets are typically quasi-rectangular with rounded corners or are even round.) During sputtering, this shape is eroded away, and by the target's “end of life” (i.e., the point at which the used target is replaced by a new pristine target), typically only a portion of the target material has been utilized. Thus, the user of the sputter target typically must discard the remaining target material (and thus most of the remaining value of the original target). As described in U.S. Patent Application Publication Nos. 2008/0216602, 2008/0271779, and 2013/0156967 (the entire disclosures of which are incorporated by reference herein), this utilization dynamic makes sputter targets good candidates for refurbishment via spray deposition, e.g., cold spray.
However, sputtering targets are typically eroded away in a manner that provides an irregular surface at the target's end of life. FIG. 2 depicts a plan view of an eroded sputtering target 200 having a typical “racetrack” (or “annular”) erosion pattern. This characteristic pattern, as well as its depth profile, is typically a result of the shape and intensity of the magnetic field applied by the magnetron during the sputtering process. The erosion pattern of eroded target 200 typically includes multiple regions of different depths. For example, the eroded target may have one or more deep pockets 210, which are regions of the most erosion (i.e., target utilization) and having the deepest surface depth. The deep pockets 210 may result from the “pinching” (i.e., increased intensity) of the magnetic field near ends 220 of the target 200, which causes erosion rates to be, e.g., 2-3× or even higher, the erosion rate in other locations of target 200. The depth of the deep pockets 210 typically determines the end of life of eroded target 200, as the target is typically replaced when this depth approaches the initial thickness (i.e., thickness prior to sputtering) of target 200. That is, the eroded target 200 is typically replaced when the bottom surface of the deep pockets 210 approaches the back surface of the target 200.
As shown, the erosion profile of eroded target 200 also includes one or more medium-depth regions 230 of less erosion and having shallower depths than those of deep pockets 210. The medium-depth regions 230 typically result from the shape of the magnetic field applied during sputtering, which tends to be less intense away from ends 220 of the target 200. The erosion profile of eroded target 200 also includes one or more shallow regions 240 from which little if any material of the target 200 is sputtered. That is, the thickness of the target 200 in shallow regions 240 may be only slightly less than, or even substantially equal to, the initial thickness of target 200 prior to sputtering (which may be, e.g., approximately 18 mm or even greater). As shown, the deep pockets 210 and the medium-depth regions 230 may collectively define at least a portion of a recessed annulus on target 200, where the deep pockets 210 correspond to opposite ends of the annulus near opposite ends of the target 200 (e.g., narrower ends of a substantially rectangular target 200). All or part of the center of the annulus may correspond to one or more of the shallow regions 240.
FIG. 3 is side view of an eroded target 200 depicting an exemplary surface contour (represented by the dashed line) extending from deep pockets 210 to the medium-depth region 230. In shallow regions 240 the target 200 has a thickness 300 that may be only slightly less than (e.g., 5% less than, or even less) or substantially equal to the initial thickness of target 200 prior to sputtering. As shown, the deep pockets 210 have depths 310 that extend deep into the thickness of target 200. For example, depth 310 may be greater than 50%, or even greater than 75%, of the initial thickness of target 200 prior to sputtering. Furthermore, the remaining thickness of the target 200 beneath deep pockets 210 (i.e., the difference between the initial thickness of target 200 and depth 310) may be, for example, in the range of 1 mm to 3 mm. Medium-depth regions 230 have considerably thicker remaining thicknesses of the target 200, and may have depths 320 of only 10%-25%, or even less, of the initial thickness of the target 200. As also shown in FIG. 3, the eroded target 200 is typically attached (e.g., bonded) to a backing plate 330 that supports the target 200 during the sputtering process and may act as a conduit for coolant (e.g., water) that regulates the temperature of target 200 during sputtering.
The irregular erosion profile of eroded target 200 presents challenges for a refurbishment process, and indeed, many eroded targets are simply recycled and replaced with new targets. Even spray refurbishment processes that selectively target the eroded racetrack pattern on target 200 can be time-consuming and expensive, and tend to require large amounts of the sprayed particulate material. Such processes may even require large spray-deposition tools and complicated robotics, and/or may require that the backing plate be removed prior to refurbishment (thus increasing the complexity, time, and expense of the process). Thus, there is a need for a refurbishment process that extends the useful life of eroded sputtering targets that may be performed quickly and inexpensively, and that does not require substantial amounts of sprayed material. Such a process would also advantageously provide refurbished targets having sputtering properties (e.g., sputter rate, sputtered film thickness, sputtered film uniformity) on par with those of the original target.